As a precautionary measure, the bank has blocked the debit cards of those accounts thought to be affected. It has also pledged to refund any monies lost through the hack. Higgins has said that the total amount stolen was a “big number but not a huge number”.

It’s not immediately clear how the hack took place. Speaking to Business Insider, Cliff Moyce, global head of financial services at DataArt, said that the timing of the attack would have helped. By orchestrating it on a weekend, the bank had less staff in the office, and was therefore less able to identify and quickly mitigate against the threat.

Morce added that the chance of the attack being a “remote technical hack” is less than fifty percent. He said that it’s far more likely the action (or inaction) of a human actor, or a sloppy data management protocols.

“Tesco will need to investigate the possibility of an ‘economic hack’ in which an offshore employee is offered multiples of their annual salary in return for a tranche of customer data. But incompetence (e.g. weak control) rather than ill intent from an employee or sub-contractor remains the more likely factor to be correlated with the malintent of the criminals.”

Several people have taken to Twitter to complain about their missing funds. Londoner Ajeet Khatri said that £2000 is missing from his account, while another Twitter user complained about a missing £1600. Many are also angry at the long wait times to speak to a customer representative, with some reporting two hour waits to speak to someone.